Current methods in the production of complementary metal oxide semiconductor (CMOS) transistors require masking layers to separate and protect active device regions such as dielectric, metal interconnect, strain, source/drain, and the like. Silicon nitride (Si3N4) or silicon oxide (SiOx, wherein x is greater than O) is often used as a masking layer due to its electrical and morphological similarity to silicon dioxide (SiO2), as well as because silicon nitride is easily bonded to SiO2. Generally, silicon nitride is used as an etch-stop layer but in certain cases, such as in a “dual damascene” process, the silicon nitride must be etched away without altering the carefully-controlled thickness of the silicon dioxide underlayer. In such instances, the etch selectivity of silicon nitride to silicon oxide, calculated as the etch rate of silicon nitride divided by the etch rate of silicon oxide, ideally is as high as possible to improve the process margin. As devices continue to shrink, the thickness of masking layers and underlayers shrink in tandem. Etch selectivity for ultra-thin layers will become more of a challenge in the future.
Current techniques for selectively etching silicon nitride may use differing chemistries and approaches. Both dry-plasma etching as well as aqueous-chemistry etch are used in the removal of silicon nitride. Aqueous chemistry materials can include dilute hydrofluoric Acid (dHF), hydrofluoric acid/ethylene glycol as well as phosphoric acid. The decision for using the different chemistries is governed by the requirement for silicon nitride etch rate and selectivity to oxide. Aqueous chemistry methods are preferable because of the reduced cost of ownership compared to dry techniques. It is well understood the silicon nitride etch rate in phosphoric acid is strongly influenced by temperature, where the etch rate rises in response to a rise in temperature. In a wet-bench configuration such as immersing substrates into a bath of aqueous phosphoric acid solution, the process temperature is limited by the boiling point of the aqueous phosphoric acid solution. The boiling point of the solution is a function of the concentration of water in aqueous phosphoric acid solution as well as the atmospheric pressure. One current method for maintaining temperature is by a feedback-loop-controller that measures the existence of a boiling state, while adjusting the addition of water volume and heater power timing interval to the bath so as to maintain this boiling state at a target temperature, (typical range of target temperatures is from 140 degrees Centigrade to 160 degrees Centigrade). When the aqueous phosphoric acid solution is heated without addition of water, the boiling point of the aqueous phosphoric acid solution rises as the water is evaporated from the solution.
Increasing the temperature of the phosphoric acid is favorable for increasing the silicon nitride etch rate for production and lower the cost of manufacturing at the expense of lower selectivity because with current phosphoric acid recirculation tanks, the consequence of allowing a high boiling point is to reduce the concentration of water. Water is critical in controlling the selectivity of silicon nitride to silicon oxide or silicon etching. Experimental evidence shows that a non-boiling state (i.e., low water content) at elevated temperature does not result in a favorable etch selectivity.
Conversely, to improve selectivity, it would be preferable to have a high concentration of water, (i.e., dilute the acid further), however this is not practical. Increasing the concentration of water in the bath reduces the boiling point of the acid mixture. At lower temperature, the etch rate of the silicon nitride falls significantly due to the strong Arrhenius relationship of the silicon nitride etch rate with temperature.
In the current art, for example, Morris, in U.S. Pat. No. 4,092,211, discloses a method for controlling within a boiling aqueous phosphoric acid solution the etch rate of a silicon oxide insulator layer which is employed in masking a silicon nitride insulator layer. The method employs the deliberate addition of a silicate material to the boiling aqueous phosphoric acid solution. In addition, Bell et al., in U.S. Pat. No. 5,332,145, disclose a method for continuously monitoring and controlling the compositions of low-solids soldering fluxes that employ a solvent with a specific gravity closely matched to the specific gravity of the flux composition. Desirable in the art are methods and systems that can maintain a high etch rate for a masking layer and also maintain a high selectivity of etching the masking layer over the silicon or silicon oxide. There is a need for batch etch treatment systems and methods and single substrate systems and methods that can meet the goals of etch rate, etch selectivity, etch time, and/or cost of ownership.